Phate, you don't need to do anything to act as a sole trader. Just make sure you fill your self assessment. Very easy these days, you can do it on-line right up to the 31st of Jan each year, no need for help as long as you take your time. Make sure you keep detailed books as you go along.
I wouldn't worry about the tax for the time being, or any official stuff. Just start advertising and trading and keep a record of money in/out. It really is that simple. When it does come to assessment time do some reading on what you can offset, its amazing what is allowed, you can throw in 10k@40ppm, give yourself a lunch, and even a laundry allowance, you should pay little tax.
solid advice